Furnace and process for the electrolysis of aluminum



G. DE VARDA April 10, 1962 FURNACE AND PROCESS FOR THE ELECTROLYSIS 0FALUMINUM Filed Jan. 7, 1955 2 Sheets-Sheet 1 E W H m R l e 2 6 E I MY FRR U C m m M R m w MA & M N ML m w R b w A A c D I. 0 EE 2 mm 3 m5 MN m0 M 2 m hu 9 1 n1 a /AZZZZ Q I .ll. M F. u ..U...| x w I My ////////7/8. |1IH\ 2 OLD ANODE SURFACE INVENTOR GIUSEPPE DE VARDA OLD ANODESURFACE was ATTORNEY J April 10, 1962 (5. DE VARDA 3,029,194

FURNACE AND PROCESS FOR THE ELECTROLYSIS OF ALUMINUM Filed Jan. 7, 19552 Sheets-Sheet 2 INTEGRATED CARBON LAYER 19 .17 1 25 12 13 27 45 I r V 4l t/ 7* E 24 Fig.5

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IN V EN TOR.

3,029,194 Patented Apr. 10, 1952 3,029,194 FURNACE AND PRGQESS F812 THEELECTRGLYSR? F ALUMHNUM Giuseppe de Verde, 8 Via San Sisto, Milan, ItalyFiled Elan. 7, 1955, Ser. No. 480,509 Qiaims priority, application ItalyJan. 19, 1954 21 Claims. (Cl. 204-67) My invention relates to theelectrolytic production of aluminum and is described in the followingwith reference to the drawings in which:

PEG. 1 is a prospective view, partly in section along the line A-A ofFIG. 4, showing a multiple furnace emplo *ing a series of bipolarinclined electrodes for the electrolysis of aluminum;

FIG. 2 is a vertical cross-sectional view of an elemental furnace,according to the invention, for the electrolysis of aluminum;

FIG. 3 is a vertical cross-section of a multiple furnace according tothe invention;

FIG. 4 is a plan view of the multiple furnace shown in FIG. 3;

FIG. 5 is an oblique view of a portion of an anode structureillustrating one way in which the anodic surface may be integrated orrenewed;

FIG. 6 is a view similar to that of FIG. 5 showing an alternative wayfor renewing the anode.

Today, aluminum is exclusively produced in electrolytic furnace eitherwith prebaked anodes or with Soderberg carbon anodes. The metal isproduced by electrolysis of a bath 1 of molten cryolite containingdissolved alumina. Alumina (A1 0 breaks up into metallic aluminum andoxygen which combines with the anodic carbon. The anodes dip partiallyinto the bath of molten cryolite and receive the current from abovethrough metallic conductors, whence the current passes over to ironstubs or nipples which intimately contact the anodic carbon. The currentpasses from the foot of the anode (i.e. from its horizontal submergedsurface), through the bath to the cathode, the latter being formed bythe furnace bottom consisting of a carbon sole with a layer of liquidaluminum on top. The current leaves the carbon bottom through ironcathodes which are in intimate contact with the cathodic carbon. Thewhole is thermically insulated on the outside with layers ofrefractories and insulating material held in place by a strong externalme al shell. The refractory lining should never come into contact withthe bath, being separated from the latter as long as the furnace is newat least, by means of carbon sides. During furnace operation anodiccarbon will be consumed, but the level of liquid aluminum increasesduring electrolysis. The level of aluminum goes down immediately upontapping. In order to maintain the proper voltage drop across thefurnace, mechanism is provided for moving the anode up and down. Thedistance between carbon anode and liquid aluminum (called the electrodespacing) is kept approximately constant and may vary in commercialaluminum furnaces from 3.5 to 9 cm., in different installations. Theelectrode spacing is oneof the peculiarities of the conventionalfurnace, and its adjustment, at first sight, seems simple enough. Whenthe alumina content in the bath drops below a certain value, theso-called anodic effect will take place and the furnace voltage willincrease suddenly from 5 v. up to between 30 and 60 v. The crust (notshown) formed by freezing on top of the bath has to be broken up bymechanical means so that a new charge of alumina may be fed into thebath to restore normal operating conditions. The new charge of aluminais always put upon the crust in advance, both to preheat it and tobetter insulate the bath. The gases evolving from the bath surface(usually CO +CO) are removed, in a closed furnace, by means of a bigexhaust system (not shown) that will also remove the volatilehydrocarbons caused by cokification of the fresh carbon paste fed fromthe top to the self'baking Soderberg anodes. lf pro-baked anodes areused, the consumed anodic parts are taken out of the bath and replacedperiodically by entirely new anodes.

In comparison with the progress attained in many other industries, onlyminor advances seem to have been made in the electrolysis of aluminum.Sufiice it to say that efficiencies, electrical ones especially, arestill sur- -risiugly low (energy efiiciency is less than 35%). Thefurnaces are still complicated and expensive, and their operatingconditions are apt to change, not only from one furnace to another but,in time, also in the same furnace.

High unit consumption of power is one of the main drawbacks of presentday furnaces. Overall power consumptions of less than 18 kwh. on directcurrent, and about 20 kwh. on alternating current, are hard to obtaineven if optimum values of about 16 kwh. per kilogram aluminum can beattained for short lengths of time.

Assuming an electrolytic decomposition voltage reuirernent of between1.3 and 1.7 v. and about 3000 ah. (the theoretical coulombs required perkg. of aluminum), theoretical per unit power consumption would be asmuch as 4 to 5 kwh. in the processes heretofore used. The averagecommercial ampere efiiciency may be assumed to be between and Thus themain source of waste can easily be traced back to the voltage drop whichis necessary to send current through the furnaces heretofore used. As amatter of fact, these furnaces are usually operated at voltages between4.5 and 6 v.

If commercial unit power consumption is to be kept within theabove-mentioned limits, large anodic surfaces have to be used to affordrunning the furnaces at reduced current density (0.6 to 1.1 21/ sq. cm.of horizontal anodic surface). On the other hand, if proper heattransfer conditions are to be maintained, very high amperages-from30,000 to 50,000 a. or more-must be resorted to. Heretofore, the trendhas been toward the adoption of very high currents in order to reduceinstallation cost and labor charges per unit production coat.

The problem of matching those high currents with the low voltages hasbeen solved, as far as electric machinery is concerned, by connectingfor instance one hundred furnaces in series and, at the same time,connecting two or more converters or rectifier-s in parallel, theamperage thus fed being, as a rule, roughly constant.

The above-mentioned shortcomings of commercial aluminum furnaces,however, increase the unit costs of the electric machinery. Not onlydoes high power consumption influence the cost of the product, but italso limits the potential output (during dry seasons or during industrial booms, for instance). The aluminum industry must today be confinedto sides where plenty of electric power, at extremely low rates, isavailable. But the aluminum industry will not be able to keep abreast ofpaying increasing power rates while a gradual industrialization of thesurrounding country takes place. For some decades past the industry hasmade but little progress as far as the basic principles of furnaceconstruction are concerned. Today it is being threatened by having itsvital power supply curtailed, as electric power rates tend to increase.

The extreme difiiculty of the problems regarding conventional furnaces,both from a technological and from an economical point of view, can berealized if the following is taken into consideration. Even if furtherreduction of the voltage of the furnaces in commercial opera- 'nomicalbasis.

tions were technically possible and economically convenient, otherproblems like thermical insulations, size, etc., would still have to befaced. It remains to be pointed out that the operating variables todayinvolved in a commercial electrolytic aluminum furnace and which have abearing on the process, are too many. It is actually very difficult tokeep them constant, or nearly so, for sufficient lengths of time. Thesame holds true for what has been defined above as the electrodespacing.

Of the above-mentioned operating variables, the three followinginterdependent groups are fundamentally important. They have always tobe kept in mind by those with whom the responsibility of furnaceoperation rests.

(a) Temperature and bath-composition (as it may be affected by theconditions of the bottoms or of the anodes, etc.)

(b) Quality and labor attendance (feeding of alumiha and carbon paste,adjustment of electrode spacing, abating the anode effect, bathcorrections, set-ting the anodic stubs, etc.)

Electrical variables such as irregular ampere distribution, currentefficiency, etc.

it is an object of my invention to greatly minimize or eliminate allabove-mentioned disadvantages by a radical change in furnace design asregards shape, arrangement, and capacity of the commercial cells, aswell as in furnace operation.

Another object of my invention is to provide a new type of furnacehaving a very low unit consumption of power as well as of other itemsaffecting the cost of the aluminum produced.

Other objects consist in limiting both direct and indirect installationcosts, in achieving greater flexibility of production and enabling thedesign of plants having far apart limits of productive capacity. It Willbe no longer necessary to reach a huge productive capacity (cg.20,000-80,000 metric tons per year) in order that new plants accordingto the invention can be run on an eco- Still other objects will becomeapparent from the following description.

In brief, the present invention contemplates the provision of a newelemental furnace having an electrically consumable but stationary anodeas well as a multiple furnace derived therefrom. The elemental furnacefor electrolytic production of Al from A1 0 is characterized by itselectrolytic cell having an inclined interspace between plane parallelfaces of two stationary carbon block electrodes arranged side by side,one anodic and the other cathodic, into which there penetrate the metalconductors (nipples) carrying the current. The metal conductors ornipples may terminate respectively at equal distances from theirelectrode faces; and the lateral walls of the interspace, as well as allthe other internalsurfaces of the furnace, are lined with anon-conductive protective layer. Below the interspace there is provideda chamber for collecting the liquid aluminum formed by. the electrolyticprocess, which chamber is provided with a tap hole. A chamber isprovided above the interspace for the gases developed between the carbonelectrodes during the electrolysis. The anodic electrode has an activelayer built up from the bath side. By stationary electrodes or blocksare meant at least two electrodes or blocks every two points of whichhave a permanent distance relationship between each other while thespacing relationship between the electrodes or blocks may vary (e.g.because of increases of the electrode spacing on account of the anodicconsumption). The term carbon is used herein to designate anycarbonaceous electrode materials such as conventional prebaked carbonanodes of amorphous carbon agglomerates, as Well as graphite, orcompositions containing a prevailing proportion of the chemical elementC or of said carbonaceous materials,

The multiple furnace is characterized by a plurality of analogous cellswith lower metal (collecting) chambers and with upper gas chambers likethose of said elemental 4 furnace, the interspaces constituting saidcells being formed by stationary carbon blocks, without metallicconductors, having cathodic and opposed built-up anodic faces, andaligned between two stationary blocks of nippled carbon arranged at theends of the multiple furnace and acting as first anode and as lastcathode respectively, for the two terminal cells.

These and other inventive characteristics are hereinafter set forth morein detail.

Structural Features of Elementary Furnace 'As schematically shown inFIG. 2, by way of example but not for limitation of my invention, thebasic elements of the furnace for the electrolysis of aluminum accordingto my invention consist of the three following parts: a practicallyhorizontal upper chamber 12 for gases; an inclined gap 13 below, whichcommunicates with the upper chamber 12 and constitutes the electrolyticcell proper and has inclined conducting main walls; and a lower chamber14 for the molten metal, which is located beneath the electrolytic cellproper and communicates with the cell, being preferably wider andshallower than the latter.

The cell is essentially made up of two electrodic carbons, one of whichmay be of graphite 15 and the other 16 of amorphous carbon agglomeratessuch as are manufactured by known methods for electrodic purposes. Bothelectrodes are planar on all sides. Both conducting faces slopedownwards, and are parallel, of substantially the same area and faceeach other. Preferably the angle of inclination of electrode faces isbetween 15 and 45 degrees with respect to the vertical. The faces areusually less than 12 cm. apart, the gap between them being preferablyfrom 4 to 8 cm. wide. As illustrated, the upper bath surface is equal toor greater than the area of the horizontal cross-section of the gap.Also the ratio between the measure of the gaps horizontal cross-sectionand the corresponding cell volume of the electrodic gap is less than0.10 cm.- and preferably less than 0.03 CHI-T1.

The current arrives at or starts from either carbon electrode throughmetallic stubs or nipples 17 of iron for example. The stubs are inintimate and extensive contact with the carbons. The ends of all stubsof each electrode are equally distant from the sloping surface whichacts as anode (carbon 16) or, respectively, as cathode (carbon 15), thedistance from said sloping surfaces being less than 50 cm. andpreferably less than 20 cm. The number and size of the stubs 17 are suchas to allow a most uniform current distribution and flux lines asparallel as possible across both electrodic surfaces.

The cell side-walls are made of one or more sufficiently bath resistant,electrical and thermal insulating materials. The inner layer 18 incontact with the moltencryolitic bath is preferably made of a solidmaterialwhich has previously been fused or sintered at extremely hightemperature (in order also that its porosity be reduced); it must resistthe bath components and be a non-conductor of the current or at least apoor one. Linings of aluminum nitride, aluminum oxide, magnesia andother known inert materials the latter preferably fused or sintered, aresuitable for those purposes. The walls, bottom and covering of the lowerchamber for the metal, are entirely lined with said inert material. Thelining is supported by a refractory layer 19 preferably of calcinedmagnesite.

After this comes a heat-insulating layer containing, pos- V The lowerchamber 14 is appreciably wider and, as a rule, shallower than the cellbetween both carbon electrodes. The volumetric content of the lowerchamber should preferably be about equal to or somewhat greater than themaximum capacity of the cell above.

In FIG. 2, the cross-section of the lower chamber istrapezoidally-shapcd, but rectangular, rhombic shapes etc., or sectionssimilar to these, may as well be adopted.

Since the furnace sidewalls are preferably higher than the carbonelectrodes, an upper chamber on top of the electrolytic gap is formed,into which gases that develop during electrolysis will evolve.

The upper chamber 12 may be closed by means of an easily removable cover(not shown in FIGURE 2). This part of the furnace, in fact, has to bereadily accessible for inspection, control and operation.

As a matter of fact, the cover, provided there is one, will insulate theupper chamber from the outside and allow for disposal, by known methods,of electrolytic gases in a more practical manner. To this end, thesidewalls of the upper chamber are provided with gas ducts (not shown inFIG. 2). Both upper and lower level surfaces etc. of the carbonelectrodes are covered with the usual layer 18 of refractory and inertmaterial such as previously described. Alumina may be spread over thetop layer both to reduce heat dispersion and to pre-heat the chargewhich has to be added to the bath at regular intervals.

The lower chamber of the metal can be reached either from above, throughthe upper chamber and electrolytic cell, or from below, through one ortwo sub-horizontal tapping channels 22 whose orifices are placed underthe head of molten aluminum 23 and the overlying cryolitic bath 24.Preferably, the volumetric capacity (for aluminum) of the lower chamber14 is 0.8 to 1.5 times the maximum capacity of the cell gap.

Operation of Elementary Furnace In the gap, which is filled with moltenbath 24 and is between the sloping and parallel faces of the twocarbons, electrolysis will take place. The bath is made up of cryolite,alumina and other well known bath-compo nents (for instance cryolitealuminum fluoride, calcium fluoride, etc.). Carbon 16 which forms, so tosea, the sloping roof of the cell, acts as the anode. The gases whichdevelop from the bath against its sloped but planar face are conveyed bythe latter out of the bath and into the upper chamber.

The second carbon acts as the cathode; upon its inclined face aluminumproduced will settle downwards in the form of little drops and/ or aveil conveyed by gravity into the lower chamber 14 whose walls havelittle or preferably no conductivity.

The metal layer 23 which collects into the lower chamber 14 can be inelectrical contact, or nearly so, with the cathodic surface of the cellby virtue of the stream of molten aluminum flowing from said surfacewithout leading to trouble.

The molten bath may be partly covered by a thin crust of frozen hath(not shown in the figures). The alumina layer on top of the bath and/ orof the electrodes for isolation or preheating, is not shown either.

For the sake of exemplification only, a description of one among manyconvenient ways to operate the present elementary furnace will now bemade, no limitation of this invention, however, being meant.

lf direct current, kept at constant value of 0.4 amp./ sq. cm., orgradually decreasing from 0.5 to 0.3 amp./ sq. cm., is sent through afurnace of this kind, whose electrodic faces are, for instance,initially about 4 cm. apart, a volage drop of 2.8 to 3.6 v. betweenanodic and cathodic bars will occur, and a power consumption amountingto from 11 to 15 kwh. per kg. of aluminum produced will result.

; The above figures apply to known baths for the elec- 6 trolysis ofaluminum, operating at known temperatures, for instance between 930 C.and l0tl0 0, provided from 3 to 6 charges of alumina per day are made,the furnace being heated from the outside by means of an independentheat source (not shown in the drawing).

When the distance between electrodes is 4 cm., the metal chamber willjust have been emptied of molten aluminum, or nearly so. Therefore, withthe possible exception of a thin layer of aluminum on its bottom, bothchamber and cell above are full of a new molten bath having, preferably,an alumina content of from 6% to 13%. As electrolysis goes on, metallicaluminum will be produced. Anodic carbon will also be consumed, as isknown. The molten metal which collects on the bottom of the lowerchamber will displace an equal volume of bath. As soon as the aluminacontent in the bath, drops below a certain percentage (eg. 3% to 5% orless) a new alumina charge, that has previously been heated in the upperchamber or on the bathcrust, will be fed into it.

On the other hand, the good heat-insulation provided by a layer ofalumina 8 to 10 centimeters thick will pre vent both heat dissipationand a tendency of the bath to form thick crusts. Alumina feeding thusbecomes an appreciably simpler operation.

The increased cell capacity due to the gradual anode consumption will beroughly compensated for by reason of the increased amount, andconsequently higher level, of metallic aluminum which collects on thebottom of the lower chamber.

After a 4 to 5 day normal run, the distance between electrodes is nolonger 4 cm. but about 8 cm. The bath level in the cell will be butslightly changed while the lower chamber will be about /1 full of metal.

The metal will be tapped as usual or through the spilling channel, whoseorifice is kept sealed during operation by means of a refractory plug;or it may be lifted successively from above, until preferably only athin layer of liquid aluminum is left upon the bottom of the lowerchamber. The molten aluminum can also be removed by a suction pipelowered down through the cell to the bottom of the lower chamber. Themetal thus obtained will be subjected to known further treatments.

After tapping, the bath level, in the case of the example, will go downto about one half the cell depth. It is however advisable to expose thewhole anodic surface by emptying the cellbut not the lower chamber ofthe molten bath. The molten bath which, in a cell about cm. wide andhaving a cell depth of 60 cm., amounts to some 20 to 22 liters, ispreferably poured into a suitable well-insulated container, to be putback. into the cell as soon as the anode has been integrated asexplained below. To prevent freezing of this removed portion of the bathduring the short interval of anodic integration, wellknown devices suchas heating ovens, etc., are resorted to.

The integration of the partially combusted anode is one of theoperations strictly peculiar to the new cell. A regular plate 40 ofelectrodic carbon (see FIG. 5) less than 12 cm. thick, and in this case,about 4 cm. thick, and as wide as the anodic face to be covered, isapplied against the consumed anode surface. Such a plate, rectangularfor instance, and measuring 80 by 70 cm., is slipped into the gapbetween the electrodes forming the cell and made to adhere to the anodein such a way that the current will not meet, during cell operation, anexcessive resistance in passing through the separating layer between oldand new anode. For this purpose Sodcrberg paste 4.1, or graphitic dustand suitable cokifiable carbonaceous binding agent, is to be spreadbeforehand over the surface of the plate to be contacted.

Instead of integrating the anode by means of one single plate, it may beadvisable to prepare, in advance, a number of rectangular strips, 42(see FIG. 6), all equally thick, to be juxtaposed onto the anodic face.They may be as long as one dimension of the anodic face,

their width being a submultiple of the other dimension.

By the expressions one dimension and other dimension it is intended toindicate that. the strips need not be disposed in the direction shown inFIG. 6. If, for instance, the anodic face is 80 by 70 cm., one mayemploy five strips about 16 cm. by 70- cm. which will of course coverthe face entirely. The single pieces are then pasted against the anodicsurface alongside one another until the old anode has been entirelycovered by the new one. Although the anode portions shown in FIGS. and 6are shown separately, it is to be understood that the above-describedintegrating procedure is carried out without removal of the electrodesfrom the furnace.

As soon as the anode has been renewed, a quantity of molten bath equalto that previously removed is poured gradually back into the cell, whiletle voltage is adjusted so that a normal heat-balance can be restoredand baking of the thin layer of binding material 41 forming themechanical and electrical connection between the new and old anode, canbe quickly completed. This having been done, the operating cycle of thecell is resumed.

A most evident feature of my invention is that in this type of cellevery mechanical device for adjusting the electrode spacing has beenabolished. On the contrary, electrode spacing in operating commercialfurnaces is critical. Therefore it is kept constant within rather narrowlimits i /z cm.). In other words, my electrodes are stationary; they arepartially combusted or consumed during operation (while electrodespacing increases), and periodically and conveniently integrated insitu.

Anodic faces, in the new cell, are virtually equal to cathodic faceswhile, in conventional cells, they are about 50% to 60% of the cathodicarea. Thus, the electric resistance in the contact layer between anodeand bath can be reduced and, at the same time, the necessary conditionsare created in order that conventional anodic current densities may beconsiderably lowered. Apart from current density (amperage per unitcross-sectional area) and current (amperage), ohmic voltage drops in theanode can be reduced to very low figures on account of the metallicstubs being stationary. Consequently their layout may be easily plannedfor most convenient dimensions.

Cathodic voltage drop will also be considerably reduced since, in thenew furnace, the cathodic electrode is no longer acting as bottom of thefurnace.

The head of molten metal in known commercial furnaces is partiallyisolated from the cathodic carbon bottom by thick bath-crusts (rich inalumina and but slightly soluble), and by carbides (originating fromirregular furnace operation, local superheatings), etc.

In order to be operated at lower anodic current densities than that nowemployed in commercial aluminum electrolysis, the above-describedfurnace may be conveniently' heated by means of an auxiliary heatsource, since heat developed by the passage of the current through theelectrodes and bath will not, as a rule, be sufi'icient. in other words,the total external surface of the furnace may actually turn out to betoo great for the small amount of kwh. or of calories/hr. to bedissipated. If heat dissipation is to be balanced with or reduced totheamount of available calories, one may operate with high currentdensities, or adequate insulation (either very bulky or particularlyefiicient) may be provided, or an additional heat source, other than theelectrolytic current, may be resorted to. External heating may be usedespecially for small furnaces of the new type herein disclosed, eg. forlaboratory pots.

Moreover, as electrolysis is carried on, anodes are consumed and the gapbetween the parallel, sloped, conductive faces of the electrodes willbecome wider. The electric resistance of the bath will increase and,consecl quently, more heat will ensue, provided current density remainsconstant. On the other hand, it is convenient to keep bath temperaturewithin prcestablished limits, determined by experience. For this purposeone may gradually reduce either the current or external heat, or re sortto other methods.

The mode of operating the cell follows the conventional practicedescribed in numerous publications, among which are the following:

(a) Chemical Engineers Handbook, John H. Perry, editor, third ed., NewYork, etc.; McGraw-Hill Book Company Inc., 1950, page 1811, left column(text and Table 26);

(12) Encyclopedia of Chemical Technology edited by Kirk and Othmer,volume 1. The Interscience Encyclopedia, Inc., New York, 1947, page 602/603;

(c) Ullmanns Encyclopiidie der technischen Chemie, 3rd edition, volume3, Urban and Schwarzenberg, Munich, 1953, pages 343 and 349;

(d) LAlluminio by Koelliker and Magnani, Ulrico Hoepli, Milan, 1930,page 186;

(e) The Electrolytic Production of Aluminum by Francis C. Frary, inJournal of the Electrochemical Society, vol. 94, No. 1, July 1948, pages32 and 34;

(1) Symposium sullelettrolisi dellalluminio-Programma e Memorie, Milan,October 12-13-14, 1953, page 27.

The temperatures employed are in the conventional range. For example,reference (a) describes a range of 900-1000 C., reference (17) anoperating range just under 1000 0., reference (0) states that thetemperature of the electrolyte should be between 930 to 950 C.,reference (e) states that the electrolytic production of aluminum from afused cryolite bath is carried out at a temperature between 950 and 1000C.

The energy consumption, 11 to 15 kwh. per kg. of aluminum produced, isdescribed above, and also the current and voltage drop. This is to befavorably compared with that of reference (a) which describes an energyconsumption of 10 kw. hr./lb. Al, which is equivalent to 22 kw. hr./kg.Al, and also with that of reference (b) which states that the grosspower requirement is about 10 kw. hr. per lb. of aluminum.

The concentration of A1 0 in the bath is also conventional. We havestated, above, that a new alumina charge is fed in when the aluminacontent of the bath drops below a certain percentage, such as 3 to 5percent. This is common practice, and is described in references (a),and

The bath composition is also conventional. As stated above, it comprisesmolten cryolite containing dissolved alumina. It is common practice toadd small amounts of other substances to increase current efliciency.Various bath compositions are disclosed in references (a),

( and (1)- Multiple Furnace With Inclined Conducting Walls My newmultiple furnace shown in FIGS. 1, 3, 4 and 7 which constitutes onevariant of the present invention also solves, in a surprisinglypractical and efficient way, the above-mentioned problems.

FIGS. 3 and 4 show a longitudinal section in a vertical plane and a planview, respectively, of my multiple furnace such as it may preferably,but not solely, be built for industrial purposes. The multiple furnacemay be schematically represented as a set of elementary furnaces, orelements of FIG. 2, from which refractory and insulating head wallstogether with their metallic conductors (bars, nipples or stubs) havebeen removed. ()nly the two ends of the multiple furnace, of course,maintain said insulating and refractory head walls with the metallicfittings conducting the current.

In FIG. 3 are shown the upper gas-chambers 12, the electrolytic gaps 13and the lower metal-chambers 14. Near one end of the elongated multiplefurnace the cathodic carbon 1'5 and, at the oppositeend, the anodiccarbon 16 are shown, both carbons being connected with their respectiveterminal bars 21 by means of iron stubs 17. The cover is fragmentarilyindicated at 2? in FIG. 1. The gas outlets are shown at 30.

The bath-resistant and insulating layer 18 will, here also, line theinside walls of the lower chambers as well as the sides of the cells.

Carbon blocks 27 spaced between the nipple-fitted end electrodes arenippleless themselves and will act as anodes on their sloping surfacesfacing the cathodic electrode 15 and as cathodes on their other sloping,and parallel surface, facing the anode 16.

Graphite, as is known, is more expensive than carbon agglomerates forelectrodes, is a better electrical conductor, will offer a greaterresistance to oxidizing gases, etc. On the other hand, it requireshigher decomposition voltage when in contact with the electrolytic bathand, consequently, a higher unit power consumption per kg. of producedaluminum. For these reasons, the intermediate electrodes 27, instead ofbeing carbon only, may be entirely of graphite or partly of graphite,viz. graphite-covered as far as the cathodic portion is concerned.

The dotted line 25 represents the contact surface between the old anodeand the newly applied one. The anode surfaces are renewed in situ whennecessary as described above in connection with FIGS. 5 and 6.

Both the refractory layer Ztl, preferably of calcinated magnesite, andthe insulating layer 19, preferably containing alumina, are enclosed inan iron casing 28.

The channels 22 at the bottom of lower chambers are normally pluggedshut with refractory material of known composition. Through thesechannels the aluminum 23 which collects in a layer underneath the liquidbath 24, may be tapped.

The lower chambers are separated from one another by means of bathprooflittle partitions 26 which are entirely made up of, or simply linedwith, the abovementioned bath and metal-resistant material 18.

Nothing need be added to what has been said above as to the shape anddimensions of the lower chambers as well as the electrolytic gaps orcells.

In FIG. 3 neither a cover, nor exhaust ducts for electrolytic gases andfor vapours which develop when the anodic binding material is baked, noralumina layers being preheated, are shown. In FIG. 7 the upper chamberof an open multiple furnace is shown. Optional, partition walls dividingsaid chamber into a number of individual compartments, one for eachcell, are not shown in the drawings, said partitions resting on theelectrodic carbons and joining obviously both sidewalls of the multiplefurnace.

Operation of the Multiple Unit Furnace In order to keep nearly constantthe total heat which is being generated in the furnace, so that balancedconditions with respect to the heat externally dissipated by themultiple furnace may be established, maintaining the temperature of theindividual baths within the limits required in practice, the sum of thegap Widths might, for instance, be kept approximately constant.

In other words, running for instance a 14 element multiple furnace, onemay operate in such a way that the gap-width of each cell differs fromthat of the one or the two adjacent cells. in most cases it is howeveradvisable, when the current intensity is kept constant, that the sum ofall the individual gap-widths be kept rather closely to a predeterminedconstant average value, not affected by time.

In particular, in the cells of a multi-cell furnace as hereinbeforedescribed, the process may advantageously be carried out, according tothis invention, with such a succession of respective electrolysisperiods that the stages of electrolysis in adjacent cells aresubstantially different from each other.

The way each element, as well as the multiple furnace as a whole,operates, is similar to the operation of the elementary furnacepreviously described. In comparison with the latter, however, notableadvantages are attained: The daily metal output is many times as much asthat of the elementary cell, in fact it may go up, for instance, frombetween 13 to 14 to between to 200 kg. of aluminum in 24 hours if a Il-element multiple furnace having electrodes of the size stated above isemployed. Moreover, a further reduction of power consumption will beobtained. The unit consumption of from 11 to 15 kwh./ kg. of aluminumgoes down to 9 to 13 kwh./ kg. of aluminum, owing to the fact that theohmic voltage drops in the nipples and in their contact with the two-endcarbons are subdivided over a greater number of elementary furnaces, andprobably owing also to other reasons.

For instance, when the operation is carried out with substantially equalanodic and cathodic current densities less than 0.8 amp/sq. cm., andpreferably less than 0.5 amp/sq. cm., the resulting voltage dropsin thecells having the nippled electrodes are less than 4.5 volts and 3.8volts, respectively; and the voltage drops in the intermediate cells areless than 4.0 volts and 3.3 volts, respectively. Under these conditions,the power consumption drops to below 16 kwh., preferably below 13 kwh.per kg. of metal output. In spite of such improved power economy,additional heat is no longer required for se curing a regulated runbecause, in the multiple furnace, the amount of power available forheat-dissipation per square meter of outside surface has considerablyincreased. It is possible to operate at a nearly constant amperagewithout upsetting the heat-balance of the single cells. Since the cellsare separated from each other by means of carbon and/or graphite platesof only few decimeters thickness and consequently fairly goodheatconductors, they will act as temperature self-regulators wheneverthe temperature in individual cells tends to increase or decreasegetting off predetermined temperature limits.

Both the multiple furnace and its related process attain the doubleadvantage of bringing ohmic drops in the anodes as well as in thecathodes down to extremely low figures, with consequent very low kwh.consumption per kg. of aluminum produced, and enabling operation withanodic current densities nearly halved as compared with the commercialdensities used in the aluminum industry. This is due to the particulararrangements of my multiple furnace, as well as the small area of theopen bath surface. These advantages, moreover, are achieved withoutresorting to excessive furnace dimensions. Thus it will be possible tokeep well within the bounds set both by eificient heat-insulation and byan economic construction cost. Overall dimensions as well as cost of themultiple furnace, as referred to daily aluminum output, will thus becomea fraction of the corresponding values of the elementary furnace.

Main Difierentiatz'ng Features of the Present Invention as Compared WithConventional Known Furnaces (1) The usual mechanical device foradjusting the distance between carbon anode and liquid. cathodic metalis dispensed with in the present invention.

Advanmges.--Constructional as well as operational simplification of thefurnace and reduction of costs. Elimination of numerous wrong, henceharmful, adjustments on the part of attendant personnel or due toinaccurate measurement of the metallic level, resulting from theindefinite and not well defined layer of metallic fogs over it,resulting from bottom irregularities, resulting from the metallicsurface being more or less convex, resulting from periodical metalwaves, etc.

(2) Minimum area of uncovered bath: Such areas, if referred to output/24hrs., are several times larger in conventional known furnaces.

Advantages-The possibility of reducing heat-dissipal 3. tion to aminimum with an open bath as well as with an alumina-covered bath. Thepossibility of better insulating the bath with a thicker layer at equalcharges. Less work to be done by the operators attending the furnace.

(3) Smaller dissipating solid surfaces: The new multiple furnacepossesses such structural and constructional features as to cutheat-losses down with respect to conventional furnaces, no overly-thickouter heat insulation layers being needed to achieve this end. My cellwill dissipate, through upper anodic surfaces, about half as much heat,for instance, as conventional ones (not even considering dissipationfrom the nipples, etc).

Advantagcs.The possibility of operating at lower total and unitamperages, reduced kWh/kg. Al consumption, greater regularity inoperation, etc. (4) The alumina charged, in the new furnace, willusually be higher than grams per square centimeter of open bath surfaceand may be even higher than gr./ sq. cm. In commercial furnaces, thelayer of alumina is usually less than 19 gr./sq. cm. thick. As alreadydescribed, this increased thickness will favorably erlect heatinsulation, without, on the other hand, the increased alumina charge perbath surface unit causing diificulties. As a matter of fact the baths inconventional known cells are not deeper than to cm. for constructional,operating and cost reasons. My furnace can easily attain bath-depths of50 cm. or more, without consideration of the bath-layer in the lowerchamber, which varies, for instance, between 10 to 30 cm. in depth.

The downward velocity of alumina must also be taken into consideration.It is low enough, as a rule (a few centimeters per minute), so that thecharge may dissolve in the bath before reaching the metal layers andsettle onto the cell-bottom of the known types of furnaces.

Adv-antages.-Incrustation of the carbon cathode, with consequenttroubles that affect commercial pots, is no longer possible in the newfurnace, even though bigger alumina charges are being fed. Even if alimited amount of alumina should settle on the cathode, it would beeasily dissolved by the bath, and, not being submerged in the bottomlayer of molten aluminum, no harm would be done.

(5) Another feature of the new cell consists in the fact that theelectrolytic gases, developing from a smaller bath-surface (per kg. ofAl output), are easier to dispose of, and their flow outwards isintensive enough to prevent leakage into the cell, during electrolysis,of the air.

(6) In commercial furnaces the current, in its substantially verticaldirection of flow, will meet several perpendicular layers of differentresistivity (anodic carbon, bath, molten aluminum, cathodic carbon). Insome layers and particularly in contact surfaces between differentlayers, resistivity will vary from point to point, thus causingnon-uniform current distribution. Local superheated areas, as well asphysical and chemical alterations, will result which will intensify theabove-mentioned phenomenon and eventually cause a lower currentefficiency, higher voltage drop, and, consequently, higher unitconsumptions, especially kwh. required per kg. of Al produced.

All the foregoing undesirable eifects do not occur in the new cell; alower anodic current density, that now equals the cathodic density, isachieved; the possibility of keeping both temperature and solving-bathcomposition well within the limits set by experience is possible; and aneasy control over cathodic surface and other items, inherent to theparticular construction and way of operating of my cell, result in thenecessary conditions for a virtually uniform current distribution andfiow through layers of resistivity varying from one to another butwelldefined and almost constant in each.

This is one of the most important advantages afforded by the presentinvention.

(7) Power consumption, in the multiple furnace, is surprisingly low;from 9 to '13 'kwh. per kg. of Al output.

12 No industrial furnace has, up to now, attained such low currentconsumption values. Such a result has been obtained by solving thefollowing three main problems on which the unit consumption can be saidto depend:

(a) Abating the voltage drop by means of bigger anodes, more uniformcurrent density, reduction of the over-voltage at the anode, as well asof the anodic and cathodic voltage-drops.

(b) Maintaining ampere efiiciency by means of appropriate aluminafeeding, sufficient cell proportioning with a view to limiting lossesthrough side-walls, insulating the metal produced in each cell from thatof the adjoining cells, a more uniform electrical flux, etc. Cells withmultiple bipolar electrodes which can be shifted in the same cell andwhich date back to the past century can be run at low voltage-drops butthey cannot be operated with satisfactory current efliciencies, owingboth to a considerable amount of current being by-passed along the sidesof the mobile electrodes and to the circumstance that the continuouslayer of molten metal collecting on the bottom acts for the greater partof the current, as an easy bridge between the anode and the cathodelocated at the ends of the cell with only two bath-crossings. Cells ofthat kind have, moreover, numerous other functional shortcomings.

(c) A stationary, practical and simple arrangement that allows a verygood heat-insulation of the furnace.

(8) The prhnary cost of the new multiple furnace with inclinedconducting walls is lower than the cost of conventional horizontalfurnaces of the same output capacity.

(9) Readjustment of the volt/ampere ratio which, in the big commercial50,000 amp. units is as low as 1/ 10,000 while, for instance in mymultiple furnace having 14 cells and being operated at 2000 amp. theratio is about 2/ 100. This results in the above-m ntioned advantages'as regards the conversion plant, the currentcarrying bus-bars, thereduction of the minimum plant capacity limit, etc.

(10) The sloping anodic face will more easily convey electrolytic gasesupwardly. This is another important advantage over level-layersfurnaces. Moreover, the commercial furnaces have to be operated at evenhigher anodic current densities with consequent considerable anodicovervoltages.

(11) The sloping cathodic face will help to convey molten aluminumdownwards; a thin liquid metal film at most, will build up over it, butin no case will a layer having a thickness measured in centimeter coverthe cathode.

Although some embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that variousmodifications may be made therein without departing from the spirit ofthe invention or the scope of the appended claims:

I claim:

1. A series multicell furnace for fused salt electrolysis of compoundsreacting by electrolysis with consumable anodes, comprising more thantwo stationary carbon electrodes, including two terminal carbonelectrodes and at least one intermediate bi polar carbon electrodedefining a plurality of individual electrolysis cells in series, eachcell having electrodic surfaces inclined to the vertical and to thehorizontal and facing each other, substantially coextensive andsubstantially parallel to each other to form a slanting laterallyconfined electrolysis gap, the inclined anode surfaces facingdownwardly, and an individual collecting chamber below each gap andcommunicating therewith, the chambers being separated by electricallyinsulating partition wall means, and current supply means connected withthe two terminal electrodes.

2. A series multicell furnace for fused salt electrolysis of aluminumcompounds reacting with consumable anodes, comprising a stationarycarbon end anode and a carbon end cathode and having respectivesubstantially congruent and substantially parallel elcctrodic surfacesinclined to the vertical and defining an intermediate space, at leastone stationary intermediate bipolar carbon electrode member disposed insaid space, each of said intermediate electrode members having a pair offaces substantially paralle. to said surfaces, said anode and saidcathode defining together with said intermediate electrodes a pluralityof inclined laterally confined gaps adapted to receive electrolyte, theinclined anode surface facing downwardly, a substantially inert, currentinsulating and heat resistant container housing said anode and saidcathode and said intermediate electrode members, said housing providinga plurality of lower insulated chambers, an individual one thereof beingbelow each of said plurality of gaps and communicating with respectiveones thereof for recei ing molten aluminum, substantially inertheat-resistant and current insulating partition means in the lower partof said housing supporting the lower end of each of the bipolarelectrodes and separating the lower portions of the cells from oneanother to define individual electrolytic cells and to provide saidplurality of insulated chambers, and means for applying a source ofcurrent only to said anode and said cathode, the end anode, cathode,intermediate carbon electrode, and gaps being in electrical series.

3. In a process for the production of aluminum by fused saltelectrolysis of a compound reacting by electrolysis with a consumableanode, in a furnace cell having stationary carbon electrodes forming aninclined laterally confined electrodic gap containing electrolyte, thegap being defined between a downwardly facing inclined anode surface ofthe anode and an upwardly facing inclined cathode surface of thecathode, the improvement which comprises supplying electric current insubstantially uniform distribution over the surfaces of the anode and ofthe cathode defining said gap, with substantially the same anodic andcathodic current densities on said surfaces, and feeding said compoundinto said furnace cell, permitting the resulting aluminum as it isforming to drain from and collect beneath said electrodic gap in aheat-retentive chamber while the evolving gases rise out of the gap,continuing this operation until the gap spacing of the stationaryelectrodes increases from initial values to predetermined maximumvalues, thereafter tapping the collected aluminum and restoring, in situfrom the bath-side, the anodic electrode on its active surface to suchdimensions as to reestablish said initial values, and repeating theelectrolysis.

4. in a process for the production of aluminum by fused saltelectrolysis of a compound reacting by electrolysis with a consumableanode, in a furnace cell having stationary carbon electrodes forming aninclined laterally confined electrodic gap containing electrolyte, thegap being defined between a downwardly facing inclined anode surface ofthe anode and an upwardly facing inclined cathode surface of thecathode, the improvement which comprises feeding said compound into saidfurnace cell, permitting the resulting aluminum as it is forming todrain from and collect beneath said electrodic gap in a heat retentivecontainer while the evolving gases rise out of the ga containing thisoperation until the gap spacing of the stationary electrodes increasesfrom initial values to predetermined maximum values, thereafter tappingthe collected aluminum and draining the electrolyte from the gap,renewing said anodic carbon by securing new anodic surface portionsthereagainst, to restore, in situ from the bath-side, the anodicelectrode on its active surface to such dimensions as to reestablishsaid initial gap spacings, refilling said gap with electrolyte andcontinually repeating the above defined steps.

I 5. In a process for the production of aluminum by fused saltelectrolysis of aluminum oxide reacting by electrolysis with aconsumable anode, which comprises feeding said aluminum oxide duringelectrolysis into a furnace cell comprising a gap inclined to thevertical, the gap being defined by a downwardly facing stationary anodiccarbon electrode and an upwardly facing cathodic carbon electrode andcontaining fused cryolite as electrolyte, the improvement comprisingcollecting below the gap the molten aluminum flowing as it is formingfrom the cathodic electrode, gradually diminishing the current intensitypassing through said fixed electrodes as the gap spacing between saidelectrodes increases due to the electrolytic oxidation of said anodiccarbon, periodically draining the collected aluminum and draining theelectrolyte from the gap, renewing said anodic carbon, in situ from thebath-side, by securing a new anodic surface portion thereagainst aftersaid electrolyte is drained, refilling said gap with cryoliteelectrolyte and continually repeating tht abovcdefined steps.

6. The process of claim 4 in which the compound is alumina and theelectrolyte is crvolite.

7. A series multi-cell furnace for fused salt electrolysis, of aluminumcompounds reacting with consumable anodes, comprising a stationarycarbon end anode and a carbon end-cathode having respectivesubstantially parallel electrodic surfaces inclined with respect to thevertical and defining an intermediate space, said endanode surfacefacing downwardly and said end-cathode surface upwardly at least onestationary intermediate carbon electrode member disposed in said space,each of said intermediate electrode members having a pair of opposed,substantially parallel faces aligned parallel with respect to saidsurfaces, said end anode and said end cathode defining together withsaid intermediate electrodes a plurality of inclined laterally confinedgaps adapted to receive electrolyte, the end anode, cathode,intermediate carbon electrode, and gaps being in electrical series, aplurality of individual, electrically insulated chambers, one below eachof said gaps and each communicating with a respective one thereof forreceiving molten aluminum separately from each gap, and means forapplying a source of current to said end anode and said end cathode.

8. The process for the production of aluminum by fused salt electrolysisof a compound reacting by electrolysis with a consumable anode,particularly of A1 0 which comprises feeding said compound duringelectrolysis into a furnace cell comprising a vertically inclined gapdefined by an upper stationary anodic carbon electrode and a lowercathodic carbon electrode and con taining a fused salt electrolyte,particularly cryolite, collecting below the gap the molten aluminumflowing as it is forming from the cathodic electrode, graduallydiminishing the current intensity passing through said fixed electrodesas the gap spacing between said electrodes increases due to theelectrolytic oxidation of said anodic carbon, periodically draining thecollected alumimum and draining the electrolyte from the gap, renevingsaid anodic carbon, in situ from the bath-side, by securing new anodicsurface portion thereagainst while said electrolyte is drained,refilling said gap with cryolitic electrolyte and continually repeatingthe above-defined steps.

9. In the process according to claim 4, said restoring of the stationaryanodic electrode comprising the steps of fixing a carbon plate againstthe anodic surface by means of a paste becoming electrically conductiveunder the action of heat, returning the fused bath to the cell, andagain setting said cell into operation whereupon the ohmic heatdeveloped by current will complete the cokification of said paste to asolid layer which unifies said integrated anode.

10. In the process according to claim 4, said restoring of thestationary anodic electrode comprising the steps of fixing a pluralityof planar strips of carbon side-byside against the anodic surface bymeans of a paste be coming electrially conductive under the action ofheat returning the fused bath to the cell, and finally setting 15 saidcell into operation whereupon the ohmic heat developed by current willcomplete the colcification of said paste to a solid layer which unifiessaid integrated anode.

11. The process according to claim 3, wherein electrolysis is carriedout in a furnace comprising a plurality of cells formed by at least onestationary intermediate bipolar having a graphite cathode portion carbonelectrode and by a stationary nippled end carbon anode and a stationarynippled end graphite cathode defining the respective gaps, with such asuccession of respective electrolysis periods that the stages ofelectrolysis in adjacent cells are substantially different from eachother.

12. The process of claim 11, wherein the sum of the gap widths of thecells of the furnace is kept nearly constant in time, said furnace beingoperated with substantially constant current intensity.

13. The process of claim 3, for the porduction of aluminum byelectrolysis of A1 6 in a fused cryolitic bath, wherein the Al O to befed to the furnace cell is preheated by the furnace heat in layershigher than 10 cm. charged above the bath surface.

14. The furnace of claim 7, wherein said carbon at least of theend-cathode and cathodic faces of the intermediate electrode members ingraphite.

15. A series multicell furnace as defined in claim 2, wherein saidhousing and partition means are made, at least in a lining layerthereof, of solid magnesium oxide pie-treated at least to sinteringtemperature.

l6. The process for the production of a metal by fused salt electrolysisof a compound reacting by electrolysis with a consumable anode, in afurnace cell having staionary electrodes forming an inclined laterallyconfined electrodic gap containing electrolyte, which comprisessupplying electric current in substantially uniform distribution,respectively, over the surfaces of the anode and of the cathode definingsaid gap, feeding said compound into said furnace cell, removing fromthe gap the resulting metal by specific gravity difference, andrestoring the anodic electrode in situ, from the bath side.

17. A multicell furnace for metal production from its oxide by a fusedsalt bath electrolysis in which anodic carbon is consumed, comprising ahousing containing electrode structures comprising a stationary anodeterminal alement and a stationary cathode terminal element spaced apartin said housing, and at least one bipolar electrode structure stationedin said furnace between and spaced from said anode and cathode terminalelements, said bipolar electrode structure having opposite anode andstationary cathode polar faces, the electrode structures providing pairsof opposite bath-facing cathodic and anodic surfaces, each pair takentogether with an intervening electrolysis gap forming a cell, therebeing at least two of said gaps providing at least two of said cells,the gaps extending generally upwardly-downwardly, means for passingelectric current serially through the electrode structures andintervening electrolysis gaps, and consequently serially through saidcells, means comprising part of said electrode structures and forming arenewal structure of carbonaceous material consumable in theelectrolysis positioned adjacent individual anode faces of saidelectrode structures, the renewal structure providing a generallydownwardly-facing, bath-facing anodic face extending at an angle to thevertical, the furnace having a lower wall and providing lowerelectrically insulating partitioning means extending from the bipolarelectrode structure to the lower wall to provide separate chambers forreception of metal formed in the electrolysis individually form eachcell, each chamber raving a tap for individual draining of the metal.

18. A multicell furnace for production of aluminum by fused salt bathelectrolysis of alumina, comprising a housing containing electrodestructures comprising a stationary anode terminal element and astationary cathode terminal element spaced apart in said housing, and atleast one bipolar electrode structure stationed in said furnace betweenand spaced from said anode and cathode terminal elements, said bipolarelectrode structure having opposite anode and stationary cathode polarfaces, the electrode structures providing pairs of opposite bathfacingcathodic and anodic surfaces, each pair taken together with anintervening electrolysis gap forming an individual, laterally confinedcell, there being at least two of said gaps providing at least two ofsaid cells, the gaps extending generally upwardly-downwardly, means forpassing electric current serially through the electrode structures andintervening electrolysis gaps, and consequently serially through saidcells, means comprising part of said electrode structures and forming arenewal structure of carbonaceous material consumable in theelectrolysis positioned adjacent individual anode faces of saidelectrode structures, the renewal structure providing a generallydownwardly-facing, bath-facing anodic face extending at an angle to thevertical, the renewal structure having a thickness less than themaximurngap width whereby it may be replaced while the said stationaryelectrodes remain in place, the furnace having a lower wall andproviding lower electrically insulating partitioning means supportingthe bipolar electrode and extending therefrom to the lower wall toprovide separate chambers for reception of metal formed in theelectrolysis individually from each cell, each chamber having a tap forindividual draining of the aluminum.

19. In a process of producing aluminum by electrolysis of alumina in afused salt bath furnace, in which an lectric current is passed seriallythrough a solid anodic surface, an intervening electrolysis gap of fusedbath, through an intervening intermediate, bipolar solid electrodeproviding opposite anode and stationary cathode polar surfaces, a secondelectrolysis gap of said bath, and eventually through a stationarycathodic surface, the anodic polar surfaces being consumable in theelectrolysis, the improvement comprising collecting and tapping theresulting aluminum below each electrolysis gap, said collecting andtapping from each electrolysis gap being separate from the collectingand tapping below adjacent electrolysis gaps, and restoring the anodicsurface adjoining each gap individually by placing a renewal means ofcarbonaceous material consumable in the electrolysis adjacent therespective anode surface, the latter remaining stationary during therenewal, the gap during the renewal being at least sufiiciently wide toadmit the renewal means.

20. in a process of producing aluminum by electrolysis of alumina in afused salt bath in which an electric current is passed serially througha solid anodic surface, an intervening elec rolysis gap of fused bath,through an intervening intermediate, bipolar solid electrode providingopposite anode and stationary cathode polar surfaces, at secondelectrolysis gap of said bath, and eventually through a stationarycathodic surface, the anodic surfaces being consumable in theelectrolysis, in which process the electrode spacing varies periodicallybetween predetermined limits, the improvement comprising collecting andtapping the resulting aluminum separately below each electrolysis gap,said collecting and tapping from each electrolysis gap being separatefrom the collecting and tapping below adjacent electrolysis gaps,periodically restoring the anodic surface adjoining each gapindividually by placing a renewal means forming a structure ofcarbonaceous material consumable in the electrolysis adjacent therespective anode surface, the latter remaining fixed in position duringthe renewal, the renewal means having a thickness less than the maximumgap wi th to facilitate the renewal.

21. In a process for the production of aluminum by fused saltelectrolysis of a compound reacting by electrolysis with a consumableanode, in a furnace cell having stationary carbon electrodes forming aninclined laterally confined electrodic gap containing electrolyte, thegap being defined between a downwardly facing inclined anode surface ofthe anode and an upwardly facing inclined cathode surface of thecathode, the improvement which comprises feeding said compound into saidfurnace cell, permitting the resulting aluminum as it is forming todrain from and collect beneath said electrodic gap in a heat-retentivechamber While the evolving gases rise out of the gap, continuing thisoperation until the gap spacing of the stationary electrodes increasesfrom initial values to predetermined maximum values, thereafter tappingthe collected aluminum and restoring, in situ from the bath-side, theanodic electrode on its active surface to such dimensions as toreestablish said initial Values, and repeating the electrolysis, thecompound being alumina, the electrolyte being cryolite.

in References Qited in the tile of this patent UNITED STATES PATENTS559,729 Lorenz May 5, 1896 1,545,383 Ashcroft July 7, 1925 1,545,384Ashcroft July 7, 1925 2,480,474 Johnson Aug. 30, 1949 FOREIGN PATENTS58,956 Germany Oct. 10, 1891 1,061,906 France Dec. 2, 1953 OTHERREFERENCES Metal Industry (London), July 26, 1946, pages 74 and 75.

1. A SERIES MULTICELL FURNACE FOR FUSED SALT ELECTROLYSIS OF COMPOUNDSREACTING BY ELECTROYLSIS WITH CONSUMABLE ANODES, COMPRISING MORE THANTWO STATIONARY CARBON ELECTRODES, INCLUDING TWO TERMINAL CARBONELECTRODE AND AT LEAST ONE INTERMEDIATE BI-POLAR CARBON ELECTRODEDEFINING A PLURALITY OF INDIVIDUAL ELECTROLYSIS CELLS IN SERIES, EACHCELL HAVING ELECTRODIC SURFACES INCLINED TO THE VERTICAL AND TO THEHORIZONTAL AN FACING EACH OTHER, SUBSTANTIALLY COEXTENSIVE ANDSUBSTANTIALLY PARALLEL TO EACH OTHER TO FORM A SLANTING LATERIALYCONFINED ELECTROLYSIS GAP, THE INCLINED ANODE SURFACES FACINGDOWNWARDLY, AND AN INDIVIDUAL COLLECTING CHAMBER BELOW EACH GAP ANDCOMMUNICATING THEREWITH, THE CHAMBERS BEING SEPRATED BY ELECTRICALLYINSULATING PARTITION WALL MEANS, AND CURRENT SUPPLY MEANS CONNECTED WITHTHE TWO TERMINAL ELECTRODES.